Intensity mapping with neutral hydrogen and the Hidden Valley simulations.

This paper introduces the Hidden Valley simulations, a set of trillion-particle N-body simulationsin gigaparsec volumes aimed at intensity mappingscience. We present details of the simulations and their convergence, then specialize to the study of 21-cm fluctuations between redshifts2 and 6. Neutral hydrogen is assigned to halos using three prescriptions, and we investigate the clustering in real and redshift-space at the 2-point level. In common with earlier work we find the bias of HI increases from near 2 at z = 2 to 4 at z = 6, becoming more scale dependent at high z. The level of scale-dependence and decorrelationwith the matter field are as predicted by perturbation theory. Due to the low mass of the hosting halos, the impact of fingers of god is small on the range relevant for proposed 21-cm instruments. We show that baryon acoustic oscillationsand redshift-space distortionscould be well measured by such instruments. Taking advantage of the large simulation volume, we assess the impact of fluctuations in the ultraviolet background, which change HI clustering primarily at large scales.